Compared with other batteries, a lithium-ion battery has the advantages of light mass, small volume, high working voltage, high energy density, high output power, high charging efficiency, no memory effect, long cycle life and the like, and is not only widely used in the fields of mobile phones, notebook computers and other digital products, but also regarded as one of the best choices for electric vehicles and large energy storage devices. At present, digital electronic products, such as smartphones and tablet computers, have increasingly higher requirements for the battery energy density, so that lithium-ion batteries for commercial use can hardly meet the requirements. A most effective way to enhance the energy density of lithium ion batteries is adopting a high capacity positive electrode material or a high voltage positive electrode material.
However, in high-voltage batteries, while the charging voltage of the positive electrode material is increased, the oxygenolysis phenomenon of the electrolyte will be intensified, thereby resulting in the battery performance deterioration. In addition, the high voltage battery has a common phenomenon of the positive electrode metal ion dissolution during use. Especially after prolonged high temperature storage of the battery, the dissolution of the positive electrode metal ion is further intensified, so that the retained capacity of the battery is low. At present, a commercialized high voltage lithium cobaltate battery of more than or equal to 4.3V generally has the problems of poor high temperature cycle performance and poor high temperature storage performance, mainly reflected in thickness swelling and great internal resistance increase after high temperature cycle, and low capacity retention after prolonged high temperature storage. The factors resulting in these problems mainly include: (1) oxygenolysis of the electrolyte. Under a high voltage, the positive electrode active material has a high oxidative activity, thereby increasing the reactivity between the positive electrode active material and the electrolyte. Furthermore, at a high temperature, the reaction between the high voltage positive electrode and the electrolyte is further intensified, so that oxygenolysis products of the electrolyte unceasingly deposit on the surface of the positive electrode, surface characteristics of the positive electrode are degraded, and internal resistance and thickness of the battery are constantly increased; (2) metal ion dissolution and reduction of the positive electrode active substance. On the one hand, at a high temperature, it is extremely easy for LiPF6 in the electrolyte to decompose, thereby generating HF and PF5. HF corrodes the positive electrode and causes the metal ion dissolution, thus damaging the structure of the positive electrode material and resulting in capacity losses. On the other hand, under a high voltage, the electrolyte is easily oxidized at the positive electrode, so that metal ions of the positive electrode active substance are easily reduced and then dissolved in the electrolyte, thereby damaging the structure of the positive electrode material, and resulting in capacity losses. At the same time, metal ions dissolved in the electrolyte easily reach the negative electrode through SEI, and are reduced to metal elementary substances after gaining electrons, thus damaging the SEI structure, constantly increasing the negative electrode impedance, intensifying the battery self-discharge, increasing the irreversible capacity, and resulting in the performance deterioration.
Chinese patent CN100585935C discloses a non-aqueous electrolyte containing a dinitrile compound, which can be used to improve the cycle performance and storage performance of batteries. Kim reported in a document (Energy & Environmental Science, 2011, 4, 4038-4045) that succinonitrile (SN) can have a complex reaction with Co ions on the surface of an LiCoO2 material, thereby effectively inhibiting the decomposition reaction of the electrolyte on its surface, and improving the cycle performance of batteries. Kim reported in a document (Applied Materials & Interfaces, 2014, 6, 8913-8920) that addition of a mononitrile or dinitrile compound in the electrolyte can enhance the thermal stability of LiCoO2 batteries due to interaction between cyano groups and Co ions. The present inventor found that succinonitrile can improve the high temperature storage and cycle performances of batteries under a high voltage, but the performances are still less satisfactory and need to be further improved.